feat(joinir): Phase 193-4 - Exit Binding Builder implementation

Implemented fully boxified exit binding generation for Pattern 3 & 4.
Eliminates hardcoded variable names and ValueId assumptions.

**New files**:
- docs/development/current/main/phase193_exit_binding_builder.md: Design document
- src/mir/builder/control_flow/joinir/patterns/exit_binding.rs: ExitBindingBuilder implementation (400+ lines)

**Key components**:
- LoopExitBinding: Maps JoinIR exit values to host function variables
- ExitBindingBuilder: Generates bindings from CarrierInfo + ExitMeta
- Comprehensive validation:
  - Carrier name mismatch detection
  - Missing carrier detection
  - Loop variable incorrectly in exit_values
- Builder methods:
  - new(): Create builder with metadata validation
  - build_loop_exit_bindings(): Generate bindings, update variable_map
  - apply_to_boundary(): Set JoinInlineBoundary host/join_outputs
  - loop_var_exit_binding(): Get loop variable exit binding
- Unit tests: 6 test cases covering single/multi-carrier and error scenarios

**Features**:
- Supports both single and multi-carrier loop patterns
- Automatic post-loop ValueId allocation for carriers
- Sorted carrier processing for determinism
- Full integration with CarrierInfo and ExitMeta from Phase 193-2

**Status**: Phase 193-4 implementation complete. Ready for Phase 193-5 integration.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/patterns/mod.rs

@@ -14,8 +14,14 @@
 //! Phase 193: AST Feature Extraction Modularization
 //! - ast_feature_extractor.rs: Pure function module for analyzing loop AST
 //! - High reusability for Pattern 5-6 and pattern analysis tools
+//!
+//! Phase 193-4: Exit Binding Builder
+//! - exit_binding.rs: Fully boxified exit binding generation
+//! - Eliminates hardcoded variable names and ValueId assumptions
+//! - Supports both single and multi-carrier loop patterns
 
 pub(in crate::mir::builder) mod ast_feature_extractor;
+pub(in crate::mir::builder) mod exit_binding;
 pub(in crate::mir::builder) mod pattern1_minimal;
 pub(in crate::mir::builder) mod pattern2_with_break;
 pub(in crate::mir::builder) mod pattern3_with_if_phi;